Spectroscopic and functional models for Hmd hydrogenase

The hydrogenases are metalloenzymes that catalyze transformations of dihydrogen. Two hydrogenase classes, [FeFe] and [NiFe] catalyze the interconversion of dihydrogen to protons and electrons. In order to facilitate electron transfer, these enzymes contain iron-sulfur clusters. More recently, a third class of hydrogenase, Hmd or alternatively [Fe]-hydrogenase, was discovered in methanogenic archaea. This enzyme catalyzes the heterolysis of dihydrogen to a proton with hydride transfer to a carbocation and does not require iron-sulfur clusters.;The structure of Hmd was reported in late 2008 and revised in 2009. The revised structure is a ferrous carbonyl fragment with appended thiolate, acyl and 2-pyridone/ 2-hydroxypyridine ligands. In an effort to confirm the structure reported for Hmd, we synthesized ferrous models containing the unique acyl ligand tethered to a donor ligand. Although Hmd active site incorporates a nitrogen heterocycle, we found phosphine to be a suitable alternative.;The addition of thioesters, derived from the reaction of o-diphenylphosphino benzoic acid with a multitude of thiols, to Fe(0) carbonyls resulted in oxidative addition of the thioester to give complexes of the type Fe(SR)(Ph2PC 6H4CO)(CO)3 with a chelating phosphine acyl ligand. These complexes readily lost CO to give a dimer of the type Fe2(SR) 2(Ph2PC6H4CO)2(CO) 3. In two cases, where R=Et or 2,6-dimesitylphenyl, we were able to show phosphine binding, prior to oxidative addition, gave products of the formula Fe[PPh2(C6H4COSR)](CO)4. With the bulky R = 2,6-dimesitylphenyl thioester, the oxidative addition reaction was completely arrested.;In the case of R = Ph, we were able to carbonylate the dimer to give the tricarbonyl monomer, which exhibited a similar IR spectrum to the CO inhibited form of Hmd. The substitution reactions of this monomer with CN- and TsCH2NC were stereoselective, similar to the enzyme. The CN - derivative was characterized by EXAFS, XANES, and IR spectroscopy and all demonstrated a remarkable similarity to CN- inhibited Hmd. Protonation of the thiolate in Hmd has been proposed, and we examined protonation of the tricarbonyl monomer. The product was unstable even at -30 ºC and the IR spectrum was found to differ greatly from the Hmd active site.;A similar method to the oxidative addition of thioesters was attempted in the addition of o-(diphenylphosphino)benzaldehyde to Fe(0) carbonyls. Although an acyl hydride intermediate was detected, the isolated product is a result of C-C coupling of two aldehyde carbons to give a tetradentate bisphosphine-bisalkoxide ligand bound to a ferrous dicarbonyl fragment. A similar coupling reaction has been reported, but our method is superior in terms of cost and yields. The Fe(P2O2)(CO)2 complex reacts with (ferrocenium)BF4 to give a 50% conversion to a complex with BF3 bound to each alkoxide. The mechanism for this reaction is proposed to involve an iron mediated F- abstraction from BF4-. The binding of Lewis acids to the alkoxides was found to be general. Addition of water to a THF solution of the bis-BF3 complex resulted in loss of the P2O2 ligand as a diol. The novel diphosphine diol was isolated in analytical purity.;Hydrogen transfer reactions mediated by 2-pyridone (2hpH) complexes are potentially relevant to the mechanism by which Hmd heterolytically cleaves dihydrogen. The complex Cp*IrCl(2hp) was reported to be an excellent precatalyst for direct dehydrogenation of secondary alcohols. We found this complex reacted with secondary alcohols or dihydrogen to give a transient complex (Cp*IrHCl(2hpH)), followed by formation of a 2hp bridged dimer of the formula [Cp*IrH)2(2hp)]+. This dimer was found to be a resting state of catalysis and not the active catalyst. In the presence of Cl-, the dimer dissociates into monomers that are highly active for dehydrogenation of secondary alcohols. (Abstract shortened by UMI.)...